The Basic Science Education Series: Matter and Molecules

University Of Alberta

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181
P234
1959
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THE BASIC SCIENCE EDUCATION SERIES

MATTER

AND MOLECULES

BERTHA MORRIS PARKER

LABORATORY SCHOOL, UNIVERSITY OF CHICAGO

Checked for Scientific Accuracy by

BRYAN F. SWAN

Laboratory School , University of Chicago

Junior High

copyright. 1947 ROW, PETERSON AND COMPANY PRINTED IN U.S.A.

Home Office: EVANSTON, ILLINOIS

ALL RIGHTS RESERVED IN ALL COUNTRIES

2 7 7 0

0F ALBERTA

Matter and Molecules

Fourteen different materials are pictured on page 2. It is
not at all hard to tell these materials apart, for each one
has certain characteristics, or properties, which make it
unlike the others. Loaf sugar, for example, is hard, white, and
sweet ; it has no smell ; it does not melt easily ; it does dissolve
easily in water. No other material pictured has this same com¬
bination of properties.

But, although each of the materials has properties of its
own, all fourteen are alike in two ways : They all take up space.
They all have weight.

All materials are alike in these same two ways. In fact, we
define a material by saying that it is something which takes up
space and has weight. Heat is not a material — it does not take
up any space or weigh anything. Light is not a material — you
could not measure it by the pint or the pound. Sound, radio
waves, electric currents, and gravity are not materials, either.

It is easy to see that all the materials in the picture take up
space. No one would expect to be able to pour milk into a glass
already full of lemonade or to put an ice cube into the space
occupied by a block of wood. It is not so easy to see that some
materials — air, for example — take up space, but there are ways,
some of which you will find later, of showing that they do.

Butter, sugar, and some of the other materials pictured are
sold by the pound — it is clear that they weigh something. No
one buys silk cloth or lemonade by the pound, but simply lift¬
ing these materials tells you that they have weight. In the
case of air and some other materials, however, people were
long in discovering that they, too, have weight.

All materials taken together may be spoken of as matter.
We can now say, then, that every kind of matter takes up
space and has weight.

3

Solids, Liquids, and Gases

The materials pictured on page 2, although they can be told
apart easily, can be grouped together in different ways. An
important way in which some of them are different from the
others is that some are solids while others are liquids. You do
not have to be told that the milk, red ink, and lemonade are
the liquids. The others are all solids.

A piece of any solid has a definite shape. A block of wood, for
example, is the same shape whether it is on a table, in a beaker,
or anywhere else. Of course, the shape of the block of wood
could be changed. It could be carved into the figure of an animal.
It could be ground into sawdust. It could be split into long, thin
pieces. But it keeps its shape until something forces it into a
different shape. And in some cases it takes a great deal of force
to change the shape of a piece of solid material. Can you imagine
tearing a silver dollar in two with your hands?

A piece of a solid material also has a size of its own. For this
reason it is possible to buy 4 yards of silk cloth, or 2 square feet
of copper, or wooden timbers 2 inches by 4 inches by 20 feet.
There is no chance that a block of wood resting in a beaker
will spread outward and upward to fill the whole beaker. There
is no chance that piling other similar blocks on top of it will
squeeze it into a much smaller space.

Solids do not, as many people think, have to be hard. Wool
and silk and modeling clay are not hard, but they are solids.
They are solids because they have a size and shape of their own.

Some solids occur in the form of beautifully shaped crystals.
Quartz, for example, occurs in six-sided crystals that come to
points at the ends. Snow crystals, with their six points, are
well known to everyone.

Liquids do not have any definite shape. On a flat surface a
liquid spreads out over the surface. In a container it takes the
shape of the container.

But liquids do have a definite size. A quart of milk poured
into another quart bottle will just fill it. Poured into a half¬
gallon bottle it will fill it exactly half full.

4

Probably, when you were thinking of which of the materials
on page 2 were liquids and which solids, the question you asked
yourself was: Which ones can be poured? All liquids can be
poured. But of course sand and granulated sugar and flour can
be poured, and they are solids. At first glance, it seems, more¬
over, that they have no shape of their own. Granulated sugar,
if poured into a cup, will spread out to take the shape of the
cup. But really the separate tiny little pieces of sugar — and of
sand and of flour — have a shape of their own.

Most liquids are wet ; that is, if you put your finger or a piece
of paper into one, enough of the liquid would stick to your
finger or the paper to make it wet. But there are exceptions.
The liquid mercury, although it can be poured like water and
although it takes the shape of a container just as water does,
is not wet. If you stick your finger or a piece of paper into a
bottle of mercury, it is just as dry as before.

The sketch below shows a surprising characteristic of liquid^.
In the experiment pictured, paper clips are dropped one at a
time into a tumbler level full of water. More than a hundred
clips can usually be dropped in before any water runs over the
edge of the tumbler. Instead of overflowing, the water piles up.
It acts very much as if there were a thin skin over the top.
This characteristic of liquids is called surface tension . Perhaps
you have heard of carrying water in a sieve. This is sometimes
possible because of surface tension. It is sometimes possible,
moreover, to make a needle float on water even though steel is
heavier than water. Surface tension may keep it from sinking.
Mercury shows surface tension even more clearly than water.
Small bits of mercury are ball-shaped because of it.

Although all the materials pictured on page 2 are either solids
or liquids, not all materials will fit into these two groups. Air
is one that will not. Carbon dioxide, stove gas, hydrogen, and
oxygen are others that will not. These materials are gases.

The left-hand picture on page 6 shows a way of making clear
that air takes up space. The flask into which the boy is trying
to pour colored water looks empty but is really full of air. The
air in the flask is holding the water out.

5

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Gases have no shape of their own. It is ridiculous to think
of making air into a model of a little animal. Gases take the
shape of any container they are in. It is hard to see that they do,
because most gases are invisible. There are, however, some
colored gases that we can see. Various tests show that invisible
gases take the shape of the containers they are in just as these
colored gases do.

The right-hand picture at the top of the page also shows that
air takes up space. It shows, too, another characteristic of air.
The tumbler was full of air to begin with; and it was pushed
straight down so that none of the air could escape. But there
is now some water in the tumbler. The air has been squeezed
into a smaller space.

A gas, unlike both liquids and solids, actually has no size of
its own. If a quart bottle full of air were emptied into a really
empty half-gallon bottle, the air would spread outward and up¬
ward to fill the whole space. Similarly, even if a space is full of
air, a great deal more air can be squeezed into it. You see this
happen with automobile tires all the time. Even though a tire is
full of air, more air can be pumped in.

Every material is a liquid, a gas, or a solid. It is now clear that
you have only two questions to ask about any material to find
out which it is : Does it have a shape of its own ? Does it have
a size of its own ? If the answer is yes to both of these questions,
the material is a solid. If the answer is no to the first and yes
to the second, the material is a liquid. If the answer is no to
both, the material is a gas.

6

Changes of State

Matter, you have seen, may be in solid, liquid, or gas form.
Another way of saying the same thing is that there are three
states of matter: solid, liquid, and gas.

When we say that a material is a solid, a liquid, or a gas, we
usually mean that it is so at ordinary temperatures. But it is
possible in many cases for gases to become liquids or solids,
for liquids to become gases or solids, and for solids to become
liquids or gases. Such changes are called changes of state.

Of all changes of state those that take place in water are
probably most familiar to you. You know that water, a liquid,
may change to ice, a solid, or to water vapor, a gas. You have
seen ice change to a liquid, and you have seen the water vapor
in the air change to drops of water on the outside of a pitcher
of cold lemonade. Perhaps you do not know that water vapor
can also change directly to a solid and that ice can change to
water vapor without becoming a liquid on the way. Snowflakes
are crystals of ice formed from water vapor, and in winter¬
time wet clothes hung out on the line may “freeze dry.”

The changing of a liquid or solid to a gas is called evaporation.
The word comes from “vapor,” another word for gas. Some
liquids evaporate faster than water. Alcohol, gasoline, and
ether are among those that do. Dry ice is one of the solids you
may have seen evaporate. It changes to a gas without changing
to a liquid on the way. The left-hand picture on page 8 shows
another solid changing to a gas. Crystals of iodine are changing
to a violet vapor.

In many cases evaporation takes place merely from the sur¬
face of a liquid. But when a liquid is heated rapidly, bubbles of
gas may form below the surface and then rise to the surface

7

and break. We say then that the liquid is boiling. The water
vapor that comes from boiling water has been given the name
of “steam. ” '

The changing of a solid to a liquid is called melting. In the
right-hand picture above, the paraffin of the candle is melting
and traveling up the wick. Butter, lard, sugar, iron, copper, and
lead are among the other solids that melt.

The changing of a liquid or a gas to a solid is called freezing.
Dry ice is made by freezing carbon dioxide, one of the gases in
the air. Granite, a common rock, is formed by the freezing of
hot, liquid rock from deep in the earth.

The changing of a gas to a liquid is called condensation. The
changing of a gas to a solid may be called condensation, too,
instead of freezing. Thus, when the water vapor of the air
changes to snow crystals, we may say either that the water
vapor condenses as snow crystals or that it freezes.

In any change of state a transfer of heat takes place. A
material freezes or condenses only when it loses heat. Evapora¬
tion and melting mean a gain in heat.

Changes of state are of great practical importance. The dia¬
gram on page 7 shows one of the many uses to which we put
them. Water is being distilled to rid it of mud and other im¬
purities. The water is first heated to boiling. The steam passes
through a condensing tube. There it is cooled by cold water
flowing around it and is changed back to water. Since the mud
and minerals in the water do not change to gases at the
temperature at which water boils, they are left behind.

8

i

Puzzles To Explain

But how are changes of state possible? How can water
freeze, alcohol evaporate, and steam condense? What makes
the differences between solids, liquids, and gases? Long ago
people began to puzzle over these questions. There were other
somewhat similar puzzling problems, too.

The girl in the right-hand picture below is adding crystals of
copper sulphate to water. The crystals are bright blue. When
they are put in water they dissolve. The crystals disappear.
You can see no solid bits of the copper sulphate at all. Even if
you looked through a microscope you would not be able to see
any bits of the copper sulphate. But it is there, as you can tell
from the blue color of the liquid.

You have watched sugar and probably many other materials
dissolve. Dissolving is quite different from melting. A solid by
itself may melt when heated. It cannot dissolve unless there is
another material present to dissolve it.

By no means all solids will dissolve in water. Sand, for
example, will not. You can easily find this out for yourself by
using filter paper. Filter paper is porous paper through which
water can go easily. If a solution of copper sulphate is poured
into a funnel lined with filter paper, the liquid which comes
through the filter paper is bright blue. The copper sulphate is
still present in the water. But if a mixture of sand and water
is poured into a funnel lined with filter paper, the water comes
through, but the sand — exactly as much as you put in — is left on
the paper. None of it has dissolved.

9

Water can dissolve many solids. It can
dissolve many gases, too. It is the best
dissolver, or solvent , known. The water you
drink is almost sure to have both air and
minerals dissolved in it.

There are other good solvents, too. Alco¬
hol, naphtha, and carbon tetrachloride are
three of them. Each one of these can dis¬
solve some materials- which water cannot.

If you added some baking soda to water
and found that it did not all disappear, it
would not mean that baking soda did not
dissolve in water. There is a limit to the
amount of another material that a given
amount of any liquid can dissolve.

But how can a solid or a gas dissolve in
a liquid?

In the first diagram in the left-hand pic¬
ture on page 9 a tiny crystal of potassium
permanganate has been dropped into a test
tube full of water. It is leaving a colored
trail as it falls, because it is dissolving in the
water. The second diagram shows the same
test tube a day later. The third diagram
shows the test tube a week later. Without
being stirred, the potassium permanganate
has spread all through the water. This ex¬
periment illustrates diffusion.

You are familiar with some examples of
diffusion. The smell of onions cooking goes
through a whole house. If a bottle of ether
is opened in one corner of a closed room,
some of it is soon in the opposite corner.

How does diffusion come about?

The pictures on these two pages show
that changes in temperature may bring
about changes in size. You have learned

10

that solids and liquids have a size of their
own. But this size changes somewhat with
changes in temperature. The milk that fills
one quart bottle will exactly fill any quart
bottle if its temperature stays the same.
But if its temperature is lowered, the milk
will shrink, or contract, and will not quite
fill the bottle. If, on the other hand, its
temperature rises, it will expand, or grow
larger, and more than fill a quart bottle.
Many other solids and liquids expand when
heated and contract when cooled.

The pictures on page 10 show that brass
expands when heated. The ball when heated
becomes too big to go through the ring.

Gases, too, expand when heated and con¬
tract when cooled. The early balloon pic¬
tured on this page rose when the air inside
it was heated. The air expanded, and some
of it escaped from the balloon. The balloon
was then light enough to be pushed up by
the surrounding air.

How is it possible for materials to ex¬
pand and contract ? A brass ball has no more
brass in it when it is hot than when it is
cold. Then how can it be bigger? How can a
quart of milk swell to more than a quart
when it is warmed and shrink to less than
a quart when it is cooled? How can air
change size with changes in temperature?

More than two thousand years ago Greek
philosophers suggested a solution for these
puzzles. Perhaps, they said, materials are
made of tiny particles with spaces between.
Their idea was more or less forgotten for
a very long time, but it was the forerunner
of our modern answer to the puzzles.

11

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Molecules

In trying to solve the puzzling problems about which you
have just been told, and other problems, too, scientists of the
last century worked out this picture of matter: All materials
are made up of unbelievably small particles, or molecules, which
are always moving. In solids the molecules are packed so close
together that each molecule keeps its position among the other
molecules ; it simply vibrates rapidly back and forth. In liquids
the molecules are close together, but they move faster and much
more freely than in solids. A molecule of a liquid does not keep
its position among the other molecules of the liquid. In gases
the molecules are far apart in comparison with their size, and
they move very fast and very freely. As they move about they
are continually bumping together and changing the direction
in which they are going. The molecules of a material attract
one another. The attraction is very slight in gases, much greater
in liquids, and still greater in solids.

At first this picture of matter — the molecular theory, it is
called — was questioned by many scientists. Molecules, if they
existed, were far too small to be seen with even the strongest
microscopes. But the theory explained so many happenings so
well that all scientists came to accept it as true. And now, with
the new electron microscope, molecules of some materials have
actually been seen.

The nineteenth century ideas of molecules have had to be
changed slightly in the case of some materials, but for our
purposes those changes are not important. Let us see now how
our puzzles can be explained in terms of molecules.

12

Solids keep their shape because their molecules attract one
another with enough force to make them do so. In liquids and
gases the attraction between the molecules is not strong enough
to give liquids and gases a definite shape.

The attraction in liquids, however, is great enough to cause
surface tension. The diagram below helps explain surface ten¬
sion. A molecule below the surface of the liquid is pulled from
all directions by the other molecules of the liquid. At the surface
the pull of the other molecules is only from below. In the experi¬
ment pictured the pull is strong enough to allow the water to
pile up for a considerable distance before it overflows.

The great attraction of the molecules in steel and wood and
other similar solids for one another explains why pieces of
these materials cannot easily be pulled apart. The speed with
which the molecules of gases move, and their slight attraction
for one another explain why gases have no definite size of their
own. The molecules spread out to fill any space there is.

Gases can be compressed easily because there are big spaces
between their molecules. The molecules of liquids and solids are
already so close together that it is not easy to push them closer.

The expanding and contracting of materials when their tem¬
peratures change are easy to explain in terms of molecules.
When a brass ball, for example, is heated, the molecules move
faster and farther and the outermost molecules are pushed out¬
ward. The ball then takes up more space. When the ball is
cooled, the molecules move less fast and far and the molecules
are pulled closer together again. The ball gets smaller.

Changes of state are easily explained, too. When water evap¬
orates, the water molecules simply move so far apart that the
liquid becomes a gas. When steam condenses, the molecules
move closer together. When iron melts, the molecules of iron
move faster and more freely. When the molten iron freezes
into solid iron again, the molecules have simply slowed down
and stopped moving freely.

A lump of sugar put into a cup of water is separated into
molecules. These molecules find their way in between the mole¬
cules of water. The sugar disappears because the separate

13

molecules are much too small to be seen. Whenever anything
dissolves, its particles go in between the particles of the material
that dissolves it.

Diffusion is explained by the fact that molecules are always
moving. When a bottle of ether is uncorked, molecules of the
ether move out between the molecules of air. In the experiment
shown in the left-hand picture on page 9 molecule movement

carries the potassium permanganate all through the water.

It is important that you do not get the idea that, because
molecules are always moving, they are tiny living things. A
molecule can move without being alive, just as a ball can fall

to the ground without being alive. Living material is made of
molecules, however, just as other materials are. You yourself
are made of molecules.

Molecules of different substances are different in size. Most
of them are much smaller than those that have been seen with
an electron microscope. Molecules of air, for example, are so
small that you breathe in billions of billions of them every time
you breathe — hundreds of times as many as you could have
counted if you had been alive ever since the world began and had
been counting every minute of your life. It is a great strain on
the imagination to try to picture the very small size of the mole¬
cules and the enormous number of these tiny particles in any
sizable amount of any material.

14

Physical Changes

When a lump of sugar is dissolved in water, it keeps enough
of its properties for us to recognize it. Although it is no longer
hard and white, it can still be recognized by its sweet taste. More¬
over, the water can be made to evaporate and leave the sugar
behind, hard and white just as it was at first. The sugar is sugar
all the while. A piece of paper can be torn into tiny bits, but
each piece is still paper. When mercury is frozen, as it can be,
in dry ice, it becomes hard, but it remains the same silvery metal.
Such changes as these are called physical changes.

The distilling of water, pictured on page 7, is a series of physi¬
cal changes. The water becomes warmer — one physical change ;
it evaporates — a second change; the vapor becomes cooler — a
third change ; and, as a final change, the vapor condenses. The
changing of iodine crystals to iodine vapor, pictured on page 8,
is a physical change. The changes pictured on pages 9, 10, and
14 are physical changes, too.

The pictures on page 14 show two of the steps in making a
glass flask. The first workman is blowing a lump of hot, soft
glass into the desired shape. The second workman is cutting
off the flask. In the end the glass is cold and hard instead of
hot and soft, and the flask is not the shape of the lump of glass.
But the glass is glass still.

Dissolving, change in temperature, expansion and contraction,
changes of state, changes in shape, and the dividing up of a piece

15

of material into two or more pieces are always physical changes.
Compressing air, as you do when you force it into an automobile
tire, rubbing out a pencil mark with an eraser, mixing coloring
matter with margarine, taking the mud out of water by filter¬
ing it, and beating white of egg until it is stiff are other examples
of physical change. In none of these changes is any material
produced which was not there in the beginning.

In physical changes the molecules of a substance may move
farther apart. They may move closer together. They may find
their way in between the molecules of another substance. The
molecules of one substance may be separated from those of
another. But the molecules themselves continue to be the same
as in the beginning.

Some changes, however, are very different. The pictures on
page 15 show two such changes.

In the left-hand picture two colorless solutions are put to¬
gether. Fine particles of a bright-yellow solid are formed. There
are so many of them that they make the whole mixture look
yellow. Here a new material has been formed — a material with
properties different from those of either of the chemicals that
were mixed together. There are now molecules that are unlike
the molecules of the materials that were mixed. A change of
this kind is a chemical change.

The right-hand picture also shows a chemical change. Here
vinegar is being added to a solution of baking soda and water.
Bubbles of carbon dioxide are making the mixture foam. The
carbon dioxide is very different from the vinegar, the baking
soda, and the water. Here again a new material is being pro¬
duced — the sign of a chemical change.

16

The Beginnings of Chemistry

Thousands of years ago people learned how to bring about
some kinds of chemical changes that were very helpful to them.
They did not understand how the changes were brought about,
but they knew that in the end they had materials quite different
from those they started with. They learned, for example, how
to change wood to charcoal. They learned how to get iron by
heating a certain kind of red rock with charcoal. Another of
their many similar discoveries was that, if dough was allowed
to stand, bubbles of gas would form in it and make it rise. The
bread made by baking the dough would be light.

For a long time no one was very much concerned about how
such changes could be explained. They were simply interested
in getting the resulting products.

But more than two thousand years ago the learned men of
Greece became interested in the whys of such changes just as
they were interested in the whys of physical changes. Aristotle,
a famous Greek philosopher, came to the conclusion that all
matter was made of four elements : fire, air, earth, and water.
One material could be changed to another, he thought, by tak¬
ing away one or more of these elements or by adding one or
more of them.

Since the Greeks took the first steps in finding out what
things are made of, we can say that chemistry began with them.
But they did nothing but talk about their ideas. The Egyptians
of a few centuries later, however, thought that they might be
able to put the Greek ideas to use. Perhaps by finding out how
to add or subtract elements they could bring about new and

17

helpful changes in materials. Perhaps they might find a way
of changing iron and other cheap metals into gold. Nothing
came of their attempts.

But the idea of changing other metals into gold did not die.
More than a thousand years ago the Arabs took up the search.
They gave their studies a name, alchemy.

Alchemy flourished in Europe in the Middle Ages. Europe’s
alchemists continued to hunt for a way of changing other
metals into gold. They also hunted for a so-called “philosopher’s
stone” that would give its wearer eternal youth. The alchemists’
work was cloaked in mystery. They used a great many mystic
symbols that no one but they themselves could understand. Some
alchemists were really trying to find out whether Aristotle’s
elements were the building stones of matter. But so many
claims of the alchemists were fraudulent that alchemy came
to have a bad reputation.

The alchemists did, however, make some helpful discoveries.
They discovered some new metals and some new drugs and
other chemicals. They designed apparatus that helped them
study materials. From their work there came, too, the idea that
there are elements out of which all materials are made, but
that, instead of being air, earth, fire, and water, they are
such things as sulphur, iron, mercury, and gold. When alchemy
was turned away from a hunt for gold and the philosopher’s
stone to experiments which would find out more about the build¬
ing blocks of matter, modern chemistry began.

18

SULPHUR

Elements, Compounds, Mixtures

The chemists of today tell us that there
are more than ninety simple substances, or
elements, out of which all matter is made.
All other materials are either compounds
of two or more elements, or mixtures.

Some elements are so rare that few
people, if any, have seen them. Others are
known to almost everyone. Listed below are
fifty of the elements. After the name of
each one is given the symbol, or sign, which
chemists use for it.

Aluminum . . .

. A1

Magnesium . .

. Mg

Antimony ....

. Sb

Manganese . .

. . . . .Mn

Argon .

. A

Mercury ....

. Hg

Arsenic .

. As

Molybdenum .

.....Mo

Barium .

. Ba

Neon .

. Ne

Bismuth .

. Bi

Nickel .

. Ni

Boron .

T>

• • • • • ■*—'

Nitrogen . . . .

. N

Bromine .

.....Br

Oxygen .

. 0

Calcium .

. Ca

Phosphorous .

. P

Carbon .

. C

Platinum . . . .

. Pt

Cesium . . . > . .

. Cs

Potassium . . .

. K

Chlorine .

. Cl

Radium .

Chromium . . .

. Cr

Radon .

Cobalt .

. Co

Selenium . . . .

. Se

Copper .

. Cu

Silicon .

. Si

Fluorine .

. F

Silver .

Gold .

. Au

Sodium .

. Na

Helium .

. He

Strontium . .

. Sr

Hydrogen . . . .

. H

Sulphur ....

. S

Iodine .

. I

Tin .

. Sn

Iridium .

. Ir

Titanium . . .

. Ti

. Fe

Tungsten . . .

. W

Krypton ....

. Kr

Uranium . . .

. U

Lead .

. Pb

Vanadium . .

. V

Lithium ....

. Li

Zinc .

. Zn

19

Until very recently chemists thought that there were only
ninety-two elements. In their laboratories, however, they have
succeeded in producing four more. The four are neptunium,
plutonium, americium, and curium. The first two of these were
named for two of the planets in our solar system. Americium
was named for America, and curium for the Curies, who dis¬
covered radium.

Three common elements are pictured at the top of page 19.
From this picture you should not get the idea that all elements
are solids. They are not. Two, mercury and bromine, are liquids.
Oxygen, hydrogen, nitrogen, and several others are gases. Many
of the solid elements and one of the liquid elements — mercury
— are metals.

In a compound, elements are joined together to form a ma¬
terial quite different from the elements it is made of. Water is
one of the commonest compounds. It is composed of oxygen
and hydrogen, two gases. Table salt is a compound of chlorine,
a poisonous greenish gas, and sodium, a poisonous metal.

Like elements, compounds may be gases, liquids, or solids. In
fact, you cannot tell by a material’s appearance whether it is
an element or a compound. You would not know from the pic¬
tures of the compounds on page 19 that they are not elements.
Not even with the strongest microscope can you see in a com¬
pound the different elements of which it is made.

The chemical names of the compounds in the picture are given
in the legend. Every compound has a chemical name which
gives a clue as to what it is made of. Manganese dioxide is made
of manganese and oxygen, lead oxide of lead and oxygen. The
nickel nitrate is made of nickel, nitrogen, and, as the “ate”
indicates, oxygen. Many compounds also have common names
just as water and salt have.

20

87.27% WATER
4.94% SUGAR

3.92% FAT
2.87% CASEIN
0.56% ALBUMIN
0.71 % MINERALS

There are many, many thousands of compounds. No one
could ever give an exact figure because chemists so frequently
find new ways of combining elements to make new compounds.

A mixture may be a mixture of two or more elements. It may
be a mixture of two or more compounds. It may be a mixture of
one or more elements with one or more compounds.

Air is one of the most common mixtures. It is a mixture of
several elements — nitrogen, oxygen, argon, krypton, and neon
among others — with carbon dioxide, a compound. There is al¬
ways some water vapor in the air, too. The pictures on these
two pages show four other common mixtures. These mixtures
are all mixtures of compounds. When we make butter we are
separating the fat from the other compounds in milk. When we
make cheese we are taking out the casein and albumin.

Among the most valuable mixtures are the alloys — mixtures
of metals. Brass (a mixture of copper and zinc) , bronze (copper
and tin), and steel (iron, carbon, and often another metal)
are the most common.

In some cases the separate materials in a mixture can be
seen. But this is not true of all mixtures. Solutions are mix¬
tures, and, as you know, dissolved materials are often completely
lost to view.

Since air, milk, and steel have been mentioned as mixtures,
it is clear that mixtures, like elements and compounds, may be
solids, liquids, or gases.

There is no limit to the number of different mixtures that can
be made. By far the greatest number of the materials around
us are mixtures. Of the materials pictured on page 2, there is
only one pure element — copper. Only the sugar and the ice (if
it was made of distilled water) are pure compounds. All the
others are mixtures.

21

Atoms

The theory of molecules explained physi¬
cal changes. It did not give a clear picture
of how an element, a compound, and a
mixture differ. It did not explain, either,
how chemical changes come about. To
answer these questions scientists worked
out the atomic theory.

The smallest particle of any element,
according to this theory, is an atom. Mole¬
cules are built of atoms. A molecule may
be made of only one atom; it may on the
other hand be made of many. In the mole¬
cule of an element all the atoms, if there
are more than one, are atoms of that
element. A molecule of oxygen, for example,
is made up of two atoms of oxygen. But in
a molecule of a compound there are atoms
of at least two kinds. Every molecule of a
compound must therefore have at least two
atoms in it.

No one has ever seen an atom. This is not
surprising since only the largest molecules
can be seen even with the electron micro¬
scope. But the atomic theory, like the
molecular theory, explains so many hap¬
penings so well that all scientists now ac¬
cept it. Scientists think they now know,
moreover, a great deal about how atoms
are joined together to form molecules.

On these pages four compounds are .pic¬
tured and their formulas given. The for¬
mula for a compound tells what kinds of
atoms and how many of each there are in
a molecule of the compound. NaCl is com¬
mon salt, or sodium chloride. The symbol

22

scientists use for an element stands for one
atom of the element. A molecule of salt is,
then, made of one atom of sodium and one
of chlorine. Si02 is quartz, or silicon diox¬
ide. For every atom of silicon there are
two of oxygen. CC14 is carbon tetrachloride,
a common cleaning fluid. In every molecule
of this material there are four atoms of
chlorine and one of carbon. HgO is mercuric
oxide, commonly called red oxide of mer¬
cury. As you see from its formula, there is
in it one atom of oxygen for every one of
mercury.

The formula for water is H20. On the
back cover of this book there are models of
five molecules — two of hydrogen, one of
oxygen, and two of water. In the models
the blue half-balls stand for oxygen atoms,
the yellow for hydrogen atoms. You can
easily pick out the models of the different
kinds of molecules.

On the front cover there are models of
the molecules of three much more complex
compounds. They are all compounds that
contain carbon, hydrogen, and oxygen. But
there are different numbers of hydrogen,
carbon, and oxygen atoms in the three
molecules. This different proportion of the
three kinds of atoms makes the three com¬
pounds quite different even though they are
all colorless liquids.

Below are the formulas for a few of the
many thousands of other compounds.

Baking soda. .NaHC03 Starch . C6H10O6

Cane sugar. .C12H22On Vinegar - CH3COOH

Carbon dioxide. .. .C02 Grain alcohol .C2H5OH

Copper sulphate CuS04 Marble . CaC03

23

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CO oo oo

MOLECULES OF
AN ELEMENT

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MOLECULES OF
ANOTHER ELEMENT

03 OD 03
CO SO OO ©0

A MIXTURE OF
THESE ELEMENTS

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A COMPOUND OF
THESE ELEMENTS

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A MIXTURE OF
TWO COMPOUNDS

oxd oo 00
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A formula like CH3COOH may surprise
you. You may wonder why it is not written
C2H402. Chemists write it as they do to
tell something about the way the atoms are
joined together in each molecule.

In a compound, although each molecule
is made of more than one kind of atom, the
molecules are alike. In a mixture there are
at least two different kinds of molecules.
The diagrams at the left will help make
this difference clear.

How does the atomic theory explain the
puzzle of chemical change ? When a chemi¬
cal change takes place, there is, according
to the theory, a regrouping of atoms. Mole¬
cules of one or more new materials are
formed in the regrouping.

Suppose, for example, a little red oxide
of mercury (HgO) is heated in a test tube.
It is broken up into the elements it is made
of. The atom of oxygen in each molecule
breaks away from the atom of mercury and
joins another atom of oxygen to form a
molecule of oxygen. The molecules of oxy¬
gen escape from the test tube. The mole¬
cules of mercury are left behind. Here is
the chemist’s shorthand way of telling
what happens:

HgO _» Hg + o2

In some chemical changes atoms of two
elements join to form molecules of a com¬
pound. When powdered iron and powdered
sulphur are mixed together and heated, the
atoms of iron join the atoms of sulphur to
form iron sulphide. Here is the story in
the chemist’s language:

Fe + S — » FeS
24

A MIXTURE OF
TWO ELEMENTS
AND A COMPOUND

The chemical changes pictured on page

15 are more complicated. Here is the story

of what happens in the first experiment

in the left-hand picture:

, HgCl + NaOH -> HgOH + NaCl
(mercuric (sodium (mercuric (sodium
chloride) hydroxide) hydoxide) chloride)

The yellow solid is the mercuric hydrox¬
ide. Do you see that in this change there is
a change of chemical partners?

When vinegar is added to baking soda,
this happens:

NaHCC>3 + CH3COOH-* CO2 + CH3COONa + H20
It would take books and books to tell of
all the chemical changes that go on about
us. The atomic theory helps explain each
one of them.

Eight Common Elements

Some elements are much more abundant
than others. The sketches at the right sug¬
gest eight of the most common: carbon,
aluminum, oxygen, hydrogen, iron, sul¬
phur, silicon, and nitrogen.

The picture on page 19 showed black
sticks of .carbon. But diamonds are carbon,
too. How hard it is to believe that any ele¬
ment could have such different forms ! The
differences come from the fact that dia¬
monds are crystals of carbon while the car¬
bon of the sticks is not in crystals.

Without carbon we could not live, for
every bit of living material in our bodies is
made partly of carbon. Carbon is a part of
the living material of every living thing.

25

All our common fuels are part carbon. Hard coal is almost
pure carbon. Soft coal, wood, gasoline, kerosene, fuel oil, and
cooking gas are largely carbon.

Other rocks besides coal contain carbon. Limestone, for ex¬
ample, is a compound of carbon — calcium carbonate.

On earlier pages you were introduced to several other com¬
pounds of carbon. There are, altogether, thousands of carbon
compounds. Some, like limestone, are solids. Some, like those pic¬
tured on the front cover of the book, are liquids. Cooking gas is,
of course, a gas. Carbon dioxide is also a gas.

Green plants take carbon dioxide and build it into sugar and
starch. Sugar and starch are in many of the foods we eat. There
are simple tests by which you can find out for yourself whether
foods contain these carbon compounds.

The right-hand picture on page 28 shows the test for
starch. When a drop of iodine is added to anything containing
starch, a purple color appears — the sign that starch is present.

The right-hand picture on page 29 shows a test for certain
kinds of sugar. Fehling’s solution, a mixture of several com¬
pounds, is used for the testing. It comes in two parts, A and B.
To test a food for sugar, add equal amounts of solutions A and
B. Heat the mixture. If an orange color results, sugar is present.

Aluminum is the most abundant of all the metals on earth.
Before the nineteenth century, however, no one had ever seen
it. It occurs in nature only in compounds. Many rocks and all
clays contain it. The finding of a cheap way of separating alum¬
inum from some of its compounds is one of the triumphs of
modern science.

Aluminum is a very useful metal because of its lightness. It
has played a very important part in the advance of aviation.
It is much used for cooking utensils, too, because it does not rust
or tarnish.

Oxygen is the commonest of all the elements. The air is about

one-fifth oxygen. Water, by weight, is eight-ninths oxygen. The
oxygen in the earth’s crust weighs as much as all the other ele¬
ments put together. You yourself are more than half oxygen.

1 J; everything around you.

26

The oxygen in water, in rocks, and in your body is joined with
other elements to form compounds. In air, on the other hand,
much of the oxygen is free, that is, not in a compound.

Free oxygen is necessary for burning. For this reason fires
must have a constant supply of air. We have to breathe free
oxygen in order to live. Otherwise the food we eat cannot burn
in our bodies and furnish us with the energy we have to have.
Aviators who go high above the earth, where the air is thin,

carry tanks of oxygen with them.

It is not easy to get pure oxygen by separating it from the
other gases in the air. Oxygen can, however, be obtained from
some of its compounds quite easily. The sketch at the bottom of
page 26 shows one way of doing so. The material in the test tube
is a mixture of two chemicals : potassium chlorate (KC103) and
manganese dioxide (Mn02). Heating the mixture drives off the
oxygen from the potassium chlorate. The oxygen bubbles up
into the bottle full of water and drives the water out.

Hydrogen looks like oxygen— they are both invisible gases—
but it has some properties that make it very different from
oxygen. It is much lighter than oxygen — in fact, it is the light¬
est of all known substances. Because of its lightness, hydrogen
was once much used in balloons. Now, however, helium is being
substituted for it whenever possible, because hydrogen can be
set on fire very easily.

There is little free hydrogen on the earth, but there are thou¬
sands of hydrogen compounds. It is hydrogen, you remember,
which is combined with oxygen to form water. Hydrogen is one

27

of the four most abundant elements in our bodies. It is one of
the elements in sugar, starch, and many, many other compounds
of carbon. Hydrogen is, moreover, always a part of the chemicals
called acids.

“Acid” comes from the Latin word for “sour.” All acids taste
sour when they are weak. Vinegar, lemon juice, green apples,
sour cherries, and grapefruit are all sour because they contain
acids. And every one of the acids contains hydrogen.

Some acids are very strong. It would not be at all safe to taste
them unless they were diluted with a great deal of water. On
this page you are shown a safe way of testing for acids. The boy
is using litmus paper — paper colored with a special kind of dye.
Some litmus paper is pink; some is blue. In acids pink litmus
paper stays pink and blue litmus paper turns pink.

Litmus paper can also be used to test for bases. Bases are the
opposites of acids. In bases pink litmus paper turns blue and blue
litmus paper stays blue. Bases always contain both oxygen and
hydrogen. In the formula for a base there is always an OH. Lye
(NaOH) is a very strong base.

The sketch on page 27 shows a way of getting hydrogen. Sul¬
phuric acid (H2SO4) is poured on bits of zinc. The zinc unites
with the sulphur and oxygen in the acid, and the hydrogen is
freed. Notice from the picture that it is collected over water
just as oxygen is.

Next to aluminum iron is the most abundant metal. Thou¬
sands of years ago men found how to get iron rather easily from

28

some of its ores. They began making tools of it. Later they
learned to make it into steel. Much of the world’s industry to¬
day depends on this metal.

The left-hand picture on page 29 shows four of the com¬
pounds in which iron occurs. Notice the different colors of the
different compounds. They help you understand how hard it is
to guess, from the look of a material, what elements it is made
of. The dark-brown material is magnetite (Fe304), the red ma¬
terial, hematite (Fe203), the green, iron sulphate (FeS04), and
the yellow, iron chloride (FeCl).

Sulphur is one of the elements pictured on page 19. Usually,
as in the picture, sulphur is a yellow powder. It may be in the
form of yellow crystals instead. It may also be a dark-brown
rubbery substance. It can disguise itself just as carbon can.

You have already met two compounds of sulphur : copper sul¬
phate and sulphuric acid. Sulphuric acid is the most important
sulphur compound. It has many uses, among them making am¬
munition and fertilizer and getting gasoline from petroleum.
This acid is sometimes called “the king of chemicals.”

Sulphur itself used to be called burning stone, or brimstone,
because it catches fire very easily. Because it is easy to set on
fire it is used in making matches. Sulphur is also used in manu¬
facturing things of rubber. Rubber was of very little importance
until the discovery was made that sulphur could be used to keep
it from being sticky in warm weather ayd stiff in cold weather.
Then a great many uses — as a material for automobile tires, for
example — were found for it.

29

Silicon is, next to oxygen, the most abundant element on
earth. But you are almost sure never to have seen it. Like alum¬
inum, it is found in nature only in compounds. You have already
found that quartz is a compound of silicon. When you know that
sand is made up mostly of tiny bits of quartz, it is clear that
silicon is very abundant.

Most glass is made of sand. Silicon, then, as a part of glass
is very common in our houses. Since the lenses of our glasses
are made of glass, silicon may be helping you read this book.

Nitrogen is another element that we could not live without.
All living material contains nitrogen just as it contains oxygen
and hydrogen. You yourself, then, are part nitrogen.

Nitrogen, like hydrogen and oxygen, is a colorless gas. It is
very abundant in the air — more than four-fifths of the air is
made of it. The nitrogen our bodies must have in order to build
the new living material needed for growth and repair does not,
however, come from the air. It comes instead from some of the
foods we eat. One reason why we need to eat such foods as milk,
eggs, meat, and cheese is that they all contain nitrogen.

Nitrogen is not a very good “joiner.” There are not nearly
so many compounds of nitrogen as there are compounds of oxy¬
gen and hydrogen. Laughing gas, which you may have been
given at the dentist’s when you had a tooth pulled, is one com¬

pound of nitrogen. Nitric acid is another. Nitric acid, like sul¬
phuric acid, is important in industry. Compounds of nitrogen
are used in explosives. Explosives play an important part in
building roads, mining coal and other such everyday work as
well as in waging war.

Controlling Chemical Changes

Chemical changes are brought about in various ways. Know¬
ing how they are brought about lets us start them when we wish
them to occur. It helps us prevent them or stop them if we do

not wish them to go on.

The picture on page 30 shows a current of electricity bring¬
ing about a chemical change. The current from the dry cells is
flowing through water to which a few drops of sulphuric acid
have been added, and is breaking the water up into hydrogen and
oxygen. The hydrogen is collecting in one tube, the oxygen in
the other. Since in a molecule of water there are two atoms of
hydrogen and only one of oxygen, twice as much hydrogen as

oxygen comes from the water.

Many of the metal things we use are metal plated. Many sil¬
ver spoons, for example, are silver only on the outside. Such
spoons are made of brass or some other rather cheap metal and
are then given a coating of silver. Electric currents are used to
do metal plating. By flowing through solutions of metal com¬
pounds, they cause the metals from the compounds to be de¬
posited on the things to be plated. It goes without saying that
if you wished to plate anything with silver you would use a
compound of silver ; if you wished to plate anything with cop¬
per you would use a compound of copper ; and so on.

Heat brings about many chemical changes. You have already
found that it is possible to get oxygen by heating certain chem-

ils. You have found out, too, that heat may cause sulphur and^^ ^

31 Fubrary of the university

iron to join to form iron sulphide. In the picture below, heat is
causing a chemical change in coal. It is producing a gas that will
burn ; coal tar, which is a liquid ; and coke, which is a solid.

You have probably accidentally allowed heat to bring about
chemical changes. Whenever food is scorched, a chemical change
has taken place in it.

Heat starts fires to burning, and burning is one of our most
important chemical changes. In starting a fire, heat starts the
elements in the fuels to uniting with oxygen. New compounds
are formed that are made up partly of oxygen. For example,
when charcoal, which is carbon, burns, it unites with oxygen to
form carbon dioxide (C02).

In many cases merely putting two materials together brings
about a chemical change. The pictures on pages 15 and 27 are
examples of chemical changes brought about in this way. The
pictures on page 31 show two other examples.

In the left-hand picture on page 31 a boy is blowing his
breath, which contains carbon dioxide, into limewater. Calcium
carbonate, a white solid, is being formed. It makes the lime-
water look milky.

In the right-hand picture on page 31 a girl is pouring a few
drops of glycerin on a pile of powdered potassium permanga¬
nate. A chemical change at once begins to take place — one that
produces so much heat that the mixture bursts into flame.

Since chemical changes may be started merely by the mixing
of two materials, it is not wise to mix chemicals you know noth¬
ing about. Careless experimenting is dangerous;

Some chemical changes will not take place unless water is
present. The girl in the picture on page 33 is starting a chemical
change by adding water to baking powder. Baking powder is a
mixture of baking soda and some acid material. So long as the
mixture is dry, no change takes place in it. But as soon as water
is added, the molecules of the two materials break up and the
parts join together in a different way to form new compounds.
One of these is carbon dioxide. When baking powder is put into
moist cake batter, bubbles of carbon dioxide form and puff
the batter up.

32

%

Light brings about many chemical changes. Our very lives
depend on chemical changes which light helps bring about in
the leaves of green plants. It is light which enables green plants
to make sugar and starch from water and carbon dioxide. The
plants are then able to make other foods from the starch and
sugar. If green plants did not make food, we could not live, for
we get all our food either directly or indirectly from them.

Other chemical changes brought about by light account for
much of the fading of colored materials. Light brings about a
chemical change in many dyes.

Still other chemical changes produced by light make possible
the taking and printing of pictures. The chemical changes take
place in the chemicals with which the films and printing paper

are coated.

Blueprints are among the simplest kinds of pictures made by
chemical changes. Blueprint paper is coated with a chemical
sensitive to light, that is, a chemical in which light will bring
about changes. The paper looks pale green, as in the picture at
the top of page 34. The chemical will dissolve in water. If the
paper is washed, all the coating will wash off and leave clear
white paper.

But if a piece of fresh blueprint paper is exposed to light,
chemical changes bring about a change in color. At the same
time the coating becomes insoluble in water. When the paper is

washed, it is clear dark blue.

A blueprint like the one pictured at the bottom
page 35 is made in this way : A leaf is laid on a piece of
fresh blueprint paper. Then the paper with the leaf on
it is held in sunlight for a short time. Light strikes
the paper where the leaf does not cover it. It
changes the chemical there. The chemical
under the leaf remains unchanged or at
least is changed very little. When the
paper is washed, the unchanged
chemical is washed off and a white
picture of the leaf is left on a blue
background.

33

of

Certain chemicals, without changing themselves, speed up
the changes in other materials. You remember that, in generat¬
ing oxygen, manganese dioxide was put with the potassium
chlorate. The heat of the flame drove oxygen from the potas¬
sium chlorate ; it did not change the manganese dioxide. But un¬
less the manganese dioxide is present it is very hard to drive
the oxygen from the potassium chlorate. There are many other
materials that act like manganese dioxide. We have a number
of them in our own bodies. We have some, for example, which
help digest the food we eat. Chemicals which help bring about
changes in other materials without being changed themselves
are called catalysts. ^

Food is often spoiled by very tiny plants which grow in it.
The plants— bacteria, yeasts, and molds— bring about chemical
changes in the foods. Yeasts, for example, break up sugar into
alcohol and carbon dioxide. Sometimes extremely poisonuous
compounds are produced when bacteria grow in food.

34

It is easy to see how some chemical changes can be prevented.
Materials in which heat is likely to bring about an undesirable
chemical change may be kept cool. Materials sensitive to light
can be protected from it. Materials which change when they are
mixed can be kept apart. Materials which change when moist
can be kept dry. It goes without saying that to prevent currents
of electricity from bringing about chemical changes in a ma¬
terial one has only to see that no electric currents flow through
the material. Many ways have been found of checking the
growth of bacteria, yeasts, and molds and thus lessening the
amount of damage to foods. Among the ways of checking their
growth are drying foods, freezing them, canning them, keeping
them cold, and preserving them with salt or sugar.

But of course we do not wish to stop all chemical changes. As
you have already found out, stopping some of the chemical
changes that go on in us and roundabout us would make it im¬
possible for us to live.

35

Date Due

See lor JL U «.* . ky v> I

1. Try for yourself the experiments pictured in this book.

2. Show with colored water that a liquid takes the shape of
any container into which it is put.

3. Watch water boil in a glass flask or beaker. Where do the

bubbles form ?

4. Find out which of these liquids evaporates fastest : ether,
alcohol, carbon tetrachloride, glycerin.

5. Get some dry ice. See for yourself that this solid evapo¬
rates without first melting.

6. Distil some muddy, salty water. You do not have to have
the kind of condensing tube shown in the picture on page 7. A
simple tube which reaches into a container for the distilled -
water will do. The container will have to be surrounded by cold
water or ice so that the steam will condense.

7. Find out whether sugar dissolves faster in hot water or
in cold water.

8. Try dissolving camphor gum in both water and alcohol.

9. Plan a way of showing that air expands when heated.

10. Watch the mercury expand when you warm the bulb of a
mercury thermometer.

11. In reference books read more about alchemists.

12. Make a collection of elements.

13. Read in The Scientist and His Tools , another unitext, the
story of how the Curies discovered radium.

14. Find out how many of the compounds on the shelves of
your science room contain oxygen.

15. Put a small part of a cake of yeast in a bottle containing
either grape juice or a mixture of sugar and water. Bubbles will
begin to come from it soon. They are bubbles of C02.

16. Make a blueprint.

17. Copperplate some small metal object. Pages 30 and 31 of
What Things Are Made Of, another unitext, tell you how.

18. From What Things Are Made Of you will get many other
suggestions for chemistry experiments. Try as many of those
experiments as possible.

36

0 181 P234 1958 BK-22 02

PARKER BERTHA MORRIS
BASIC SCIENCE EDUCAT10

SERIES

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